MARYLAND — The morning after Isabel, we woke to find that the old locust tree along the side of our yard had sagged across the fence into the scraggly pine of our next-door neighbor. When we took his chainsaw to the locust, it knocked the pine down too. Legally, the mess was his problem. But there’s neighbor law and there’s neighborliness. Plus he has no fireplace, so I could have the wood. So both couples set to clipping and bundling branches, sawing the trunk, and tossing the sections into my yard. Then I split the logs and stacked them.

Good thing Isabel didn’t come last year, when I weighed 50 pounds more than I do now and had the endurance of Keats. And a good thing I’d been lifting weights all year as part of my regimen. September’s sudden timber bonanza was the payoff to all that training.

There are two kinds of fitness, broadly speaking: aerobic and anaerobic. Aerobic fitness is cardiovascular fitness — heart-lung conditioning, endurance, steady-state work capacity. Anaerobic fitness is strength, power, effort that comes in bursts.

For many years, health experts favored aerobic fitness over anaerobic. Anaerobic fitness was for bodybuilders and other steroid cases, important if you’re a blacksmith or a football player maybe, but who is? Aerobic fitness meant healthy hearts, clean lungs and lean runner’s physiques, low body-fat levels and (maybe) longer, healthier lives in a country where hardly anyone needs to hew a livelihood out of the ground by main force.

Recently, though, even health experts have come to recognize the value of anaerobic strength. Precisely because ours is a more sedentary existence than that of our forebears, occasions where you need to exert brute force have a way of sneaking up on you. You won’t move the refrigerator every day, but suddenly you have to move the refrigerator. You have to push your car, lift one end of the couch, wrestle a cooler into the trunk, carry your daughter a mile, or disassemble two trees in a weekend. You either have anaerobic strength when you need it, or you don’t. You can clip that branch or not. Helping my brother move a few years ago sent me to outpatient surgery with bilateral hernias. Post-Isabel, I tossed chunks of wood the old me couldn’t have lifted.

Even Kenneth “Aerobics” Cooper now counsels the importance of weight training, the royal road to anaerobic fitness. He recommends increasing the proportion of strength training in your fitness routine as you age, from 20 percent of your exercise time in your thirties to 45 percent in your sixties.

NOW CONSIDER A COMMON COMPLAINT against sport-utility vehicles: Most people who buy them don’t need that much power. The argument goes, the bulk of SUV purchasers are suburbanites and city-dwellers who drive on good roads and know the back-country mainly from Animal Planet and the Outdoor Life Network. What most drivers need, say critics, is a vehicle that gets much, much better gas mileage.

A comparison with personal fitness is suggestive: SUVs are anaerobic strength vehicles; high fuel-efficiency cars are aerobic. Vehicle power is like muscle power: When you need it, you need it. Maybe you have to cart a new refrigerator home. Maybe your area reliably gets one bad snowstorm a year. (In the D.C. metro area, whenever a snowstorm hits, the call goes out for SUV and four-wheel drive owners to ferry hospital workers to their jobs.) Maybe you go camping twice a year or once a week transport supplies to your Cub Scout pack.

You might argue that, while you can’t rent muscles, you can rent a car. But while some of the occasions where people need the traction and power of an SUV can be predicted, others cannot. You can’t necessarily count on the rental market to provide the SUV power you need when you need it. And it costs money to rent a car, of course. Enterprise’s website tells me that a one-week rental starting November 15 will set me back at least $423.69 with tax. (“Additional surcharges, local taxes, etc. may apply.”) Mark at the local Enterprise location tells me over the phone that I may not take my rental off-road, nor may I tow anything. For $11/day, I can get collision insurance to cover the first thousand dollars of any damage only (which takes the weekly rental above $500). Anything above a grand goes against my own collision policy, if I have one.

If you ever want to haul a trailer, or drag a boat, or take a wilderness vacation even once a year, a rental won’t do. If you find a vacation site you can legally drive to per Enterprise’s terms and conditions, the cost will be equivalent to an extra $40 plus per month, spread over a year. You’ll face the risk of damaging the vehicle — you’re renting it for heavy duty, after all — and that risk may be heightened. Because you’ve chosen not to buy an SUV, you’re not used to driving one.

You have a conditioning problem, in other words, even if your circumstances allow for the rental option. You have a worse conditioning problem if they don’t, since then you either have to attempt the same task with too little car or not make the attempt at all. In terms of expense and hassle, it only takes a few days’ worth of SUV need per year — a few trips to the dump, the nursery or the appliance store — to make the SUV ownership option look cost-effective and sensible.

As with physical fitness, there is value in maintaining the capacity for marginal exertion well beyond the daily norm. And as with physical fitness, having the extra power available may inspire you to change in ways you didn’t anticipate — you do more because, well, you can. My neighbor and I saved hundreds of dollars on a tree service because we were up to clearing the mess ourselves, and he hauled several bundles of brush to the dump in his Toyota Highlander.